Covering metal catalyst surfaces with thin two-dimensional oxide materials can enhance chemical reactions

Physically confined spaces can make for more efficient chemical reactions, according to recent studies led by scientists from the U.S. Department of Energy's (DOE) Brookhaven National Laboratory. They found that partially covering metal surfaces acting as catalysts, or materials that speed up reactions, with thin films of silica can impact the energies and rates of these reactions. The thin silica forms a two-dimensional (2-D) array of hexagonal-prism-shaped "cages" containing silicon and oxygen atoms.

"These porous silica frameworks are the thickness of only three atoms," explained Samuel Tenney, a chemist in the Interface Science and Catalysis Group of Brookhaven Lab's Center for Functional Nanomaterials (CFN). "If the pores were too tall, certain branches of molecules wouldn't be able to reach the interface. There's a particular geometry in which molecules can come in and bind, sort of like the way an enzyme and a substrate lock together. Molecules with the appropriate size can slip through the pores and interact with the catalytically active metal surface ."

"The bilayer silica is not actually anchored to the metal surface," added Calley Eads, a research associate in the same group. "There are weak forces in between. This weak interaction allows molecules not only to penetrate the pores but also to explore the catalytic surface and find the most reactive sites and optimized reaction geometry by moving horizontally in the confined space in between the bilayer and metal. If it was anchored, the bilayer would only have one pore site for each molecule to interact with the metal." The scientists are discovering that the confined spaces modify different types of reactions, and they are working to understand why.

An illustration of physically confined spaces in a porous bilayer silica film on a metal catalyst that can be used for chemical reactions. Silicon atoms are indicated by the orange circles; oxygen atoms by the red circles. Nanoconfinement can occur in the pores (zero-dimensional, or 0-D) and the interface-confined region between the film and the metal (two-dimensional, 2-D). Credit: Brookhaven National Laboratory Read more

A schematic showing how oxidation of carbon monoxide (CO) on palladium (Pd) under a 2-D microporous silica (SiO,2) cover produces 20 percent more carbon dioxide (CO 2 ), as compared to the reaction on bare Pd. This interfacial microenvironment fosters a higher coverage of reactive Pd surface oxides that are key to converting CO to CO 2 . Credit: Brookhaven National Laboratory Read more

Growth and characterization of a bilayer silica film using a low-energy electron microscope (LEEM) with full-field imaging. This type of microscopy allows scientists to follow changes in the structure of the film as it's growing in real time. Figure (a) shows a clean palladium surface imaged with LEEM (large sphere) and its accompanying electron diffraction pattern (small sphere). Figure (b) shows the imaging and diffraction patterns for bilayer silica (SiO2) grown on palladium. Credit: Brookhaven National Laboratory Read more

An illustration of the impact of bilayer silica on biomass conversion. Bulky biomass molecules such as furfuryl alcohol can only infiltrate the silica film at pore defect sites to interact with catalytically active palladium. Once trapped below the silica cover, furfuryl alcohol can break down into several derivatives, notably propane, which is difficult to produce on the open surface. Credit: Brookhaven National Laboratory Read more